In the field of copying machines of the thermal transfer type, color copiers for fixing color toners tend to increase in number. Although fixing temperatures around 300.degree. C. are required of the fixing rolls for attaining good fixation of toners, there has not been such a highly heat-resistant fixing roll for 300.degree. C. use, and the use temperature for the conventional Teflon-coated fixing rolls is around 200.degree. C. at the most.
Thus, a fixing roll which thermally fixes toners to a receiving sheet plays a very important role in copying machines because it fuses the toners and this governs the clearness and quality of the copy. On the other hand, besides the trend toward the production of color copiers, there are general trends in thermal fixing type copying machines toward size reduction and performance elevation (increase in printing speed). Under these circumstances, fixing rolls have come to be more frequently required to have better heat and abrasion resistance. Since Teflon coating film-covering rolls have insufficient abrasion resistance, the attainable minimum thickness for the coatings is about 20 .mu.m at the most. Such large coating thicknesses, however, result in a significantly impaired efficiency of heat conduction during toner fusion that has been conducted with the aid of heat, so that the number of copies that can be turned out per minute is reduced.
As a result, there are limitations in the development of large-sized copying machines of the high speed revolution type. On the other hand, in the case of small-sized copying machines so-called "stand-alone", the conventional Teflon-coated rolls should be replaced with fresh ones at intervals of 200,000 copies because of the rapid wear of the roll coatings. Accordingly, it is necessary to station persons for roll replacement at offices of copying machine users, which is very costly and can be a factor that increases the relative cost of the copying machines. For these reasons, the coating layer of a thermal fixing roll is required to have good abrasion resistance as well as heat resistance.
By the way, polybenzimidazole was developed by the late Prof. Maryell and co-workers in Arizona State University, U.S.A. and disclosed in H. Vogel and C. S. Marvel, J. Polym. Sci., Vol. 50, p511 (1961). However, their synthesis of polybenzimidazole failed to yield solvent-soluble polymer because of crosslinking reaction and hence, it is thought that use of their polybenzimidazole as, for example, a varnish for heat-resistant rolls is difficult.
Hoechst Celanese Corp. of America has succeeded in inhibiting the crosslinking reaction to yield a polymer which is available under trade name "Celazole". Reference can be made to E. J. Powers and G. A. Serad, "History and Development of Polybenzimidazoles" presented at the Symposium on the History of High Performance Polymers, American Chemical Society, New York, April 15-18, 1986 and published in High Performance Polymers. This polymer is superior in heat resistance to polyimides and currently has the best heat-resisting properties. Specifically, the polymer has a heat distortion temperature of 435.degree. C. and an oxygen index of 58%, is incombustible in the air, retains its physical properties over a wide temperature range of from a temperature as high as 760.degree. C. to a temperature as low as -200.degree. C., and has a dielectric breakdown voltage of 20.9 KV/mm, which data show that the polymer is incombustible in the air. However, there have so far been no reports on the ability of the polymer to form a film by the solvent-cast method, or on an actual examination of properties of a film formed by applying the polymer on a roll. This is attributable to the poor film-forming ability of polybenzimidazole.
Illustratively stated, since the glass transition temperature (T.sub.g) of polybenzimidazole is 427.degree. C., it is necessary for film formation that baking should be performed at a temperature of 410.degree. C. or higher. Although crosslinking proceeds only when the baking temperature is not lower than that, the crosslinking reaction competes with oxidative decomposition at such a high temperature and the control of the reaction is therefore difficult. In other words, it is extremely difficult to enhance film strength by baking only and it is expected that PBI film formation is considerably difficult.